#include <iostream>
#include <vector>
#include <cmath>
#include <map>
#include <set>
#include <iomanip>
#include <queue>
#include <algorithm>
#include <numeric>
#include <deque>
#include <complex>
#include <cassert>

using namespace std;

int main(){

    int H, W, h, w, nh, nw;
    char mc;
    string ans="";
    cin >> H >> W;
    vector<string> S(H);
    
    int dx[2] = {1, 0};
    int dy[2] = {0, 1};
    vector<pair<int, int>> v;

    for (int i=0; i < H; i++) cin >> S[i];
    ans += S[0][0];
    v.push_back({0, 0});

    for (int i=1; i<H+W-1; i++){
        vector<tuple<char, int, int>> c;
        for (auto [h, w] : v){
            for (int dir=0; dir < 2; dir++){
                nh = h + dx[dir];
                nw = w + dy[dir];
                if (nh < 0 || nh >= H || nw < 0 || nw >= W) continue;
                c.push_back({S[nh][nw], nh, nw});
            }
        }
        v.clear();
        sort(c.begin(), c.end());
        tie(mc, h, w) = c[0];
        ans += mc;
        for (auto [cc, h, w] : c){
            if (cc == mc) v.push_back({h, w});
        }
    }

    cout << ans << endl;

    return 0;
}